Boiler

ABSTRACT

The invention provides a boiler provided with low cost means which can reduce a concentration of CO, an unburned matter, an attached ash and the like near a side wall and maintain a combustion state well with a simple structure. In a boiler having a combustion chamber  13  formed by front and rear walls (burner walls) provided with a plural stages of burners  2, 3  and  4  on at least one of them and opposing to each other, and side walls  1   a  and  1   b  crossing to said burner walls  14   a  and  14   b,  a gas port  6  containing no fuel for making a pressure of a gas near said side walls  1   a  and  1   b  within said combustion chamber  13  higher than a pressure of a gas at a center portion of said combustion chamber  13  is provided between an outermost burner and the side walls  1   a  and  1   b  within a range of a height of said burner stages  2, 3  and  4.  A combustion gas  16  can not come close to the side walls  1   a  and  1   b  due to the jet  18  of the gas.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a boiler, and more particularly,to a boiler which is preferable for reducing a concentration of CO, anunburned matter, an attached ash and the like near a side wall of afurnace.

[0003] 2. Description of the Prior Art

[0004] In order to improve a heat efficiency of a boiler, it isnecessary to reduce a concentration of a carbon monoxide (CO) and anunburned matter within a furnace. In order to reduce the concentrationof CO and the unburned matter within the furnace, there has been knownthe following method.

[0005] A first method corresponds to a method of adjusting an operationcondition, in particular, a method of adjusting an air flow amount in aburner and an air flow amount in an after air port for a two stagecombustion.

[0006] A second method corresponds to a method of supplying an air to aspace in which an unburned matter is increased. As an example of thesecond method, a method of supplying an air along a wall of a furnace isshown in Japanese Utility Model Unexamined Publication Nos. 59-92346 and2-122909, and Japanese Patent Unexamined Publication Nos. 62-131106 and3-286918.

[0007] Among these conventional examples, in Japanese Utility ModelUnexamined Publication Nos. 59-92346 and 2-122909, and Japanese PatentUnexamined Publication No. 3-286918, there is disclosed a boiler inwhich an air port is provided in a lower portion of a burner stage.

[0008] In Japanese Patent Unexamined Publication No. 62-131106, there isdisclosed a boiler in which the air ports are provided on four walls ofthe furnace and the air ports are provided on upper and lower portionsand an intermediate height of a plurality of burner stages.

[0009] Inventors have verified an effectiveness of the conventionalfirst and second methods mentioned above on the basis of a measurementand a numerical analysis of an actual boiler. As a result, it has becomeapparent that the concentration of CO and the unburned matter in thecombustion gas have been still high near the side wall crossing to thewall having the burner at least at a height of the burner stage, evenwhen any of these methods is employed. Further, it has become apparentthat the ash is attached to the side wall in the case of burning a coal.

[0010] The reason is that the combustion gas generated from the burnercomes near the side wall crossing to the wall having the burner sincethe pressure near the side wall is lower than that of the combustionarea at the center of the furnace.

[0011] A countermeasure thereof is shown in Japanese Patent UnexaminedPublication No.7-98103. In this example, there is suggested a boilercomprising a plurality of burners and a plurality of air inlet ports fora two stage combustion disposed downstream of the burners, which isstructured such that an auxiliary combustion port for supplying a gasfor combustion having an oxygen partial pressure of 10% or less isprovided between a side wall of a furnace and a burner so as to adjustan injection amount of the gas for combustion injected from theauxiliary combustion port and a direction of a jet, thereby preventing aburner jet from returning to the side wall of the furnace.

[0012] However, in this prior art, a pipe for supplying the gas forcombustion having the oxygen partial pressure of 10% or less to theauxiliary combustion port is required. Since it is necessary to arrangea pipe for supplying the gas for combustion having a length of aboutsome tens meters, a great cost increase can not be avoidable.

SUMMARY OF THE INVENTION

[0013] An object of the present invention is to provide a boilerstructured such as to prevent a combustion gas from coming near a sidewall by using an air, an oxygen, a combustion exhaust gas and the like.

[0014] The present invention provides a boiler comprising a combustionchamber formed by front and rear walls and a side wall crossing to saidfront and rear walls and a plural stages of burners placed on at leastone of said front and rear walls, in which in order to make a pressureof a gas within said combustion chamber higher in a portion near theside wall than at a center portion of said combustion chamber, a gasport is provided between an outermost row burner and said side wallwithin a range of a height of said burner stages.

[0015] The present invention also provides a boiler comprising acombustion chamber formed by front and rear walls and a side wallcrossing to said front and rear walls and a plural stages of burnersplaced on at least one of said front and rear walls, in which in orderto make a pressure of a gas near said side wall within said combustionchamber higher than a pressure of a gas at a center portion of saidcombustion chamber, a gas jet port is provided in said side wall withina range of a height of said burner stages.

[0016] The present invention further provides a boiler comprising acombustion chamber formed by front and rear walls and a side wallcrossing to said front and rear walls, a plural stages of burners placedon at least one of said front and rear walls and an after air port for atwo stage combustion disposed downstream said burner stages, wherein atleast one stage gas jet port for making a pressure of a gas near saidside wall within said combustion chamber higher than a pressure of a gasat a center portion of said combustion chamber is provided between anoutermost row burner and said side wall within a range of a height ofsaid burner stages and a plural stages of gas jet ports are providedbetween said lowermost stage burner and said after air port.

[0017] In each of the boilers mentioned above, it is desirable that saidgas port is provided at portions of said opposing front and rear walls,said portions having the same height, and wherein gas supply means forinjecting said jet at a speed at which a gas jet from said opposing gasport collides in the middle of said front and rear walls is provided.

[0018] The present invention, more particularly, provides a boiler ascited in any one of the structures mentioned above, comprising supplymeans for supplying a pulverized coal as a fuel and an air fortransferring said pulverized coal to said plural stages of burners, andsupply means for supplying an air for combustion to said plural stagesof burners and supply means for supplying a gas for jetting to said gasport, in which there is provided control means for controlling a flowamount of the jet from said gas port on the basis of a load demand ofsaid boiler and a coal type information so as to reduce a flow amount ofthe jet from said gas port when a load of said boiler is low andincrease a flow amount of the jet from said gas port in accordance thatthe load of said boiler becomes higher.

[0019] The present invention further provides a boiler as cited in anyone of the structures mentioned above, comprising supply means forsupplying a pulverized coal as a fuel and an air for transferring saidpulverized coal to said plural stages of burners, and supply means forsupplying an air for combustion to said plural stages of burners andsupply means for supplying a gas for jetting to said gas port, in whichmeasurement means for measuring a concentration of a carbon monoxide(CO) in a combustion gas near said side wall is provided, and there isprovided control means for controlling a flow amount of the jet fromsaid gas port on the basis of a load demand of said boiler and ameasured result of said concentration of CO so as to reduce a flowamount of the jet from said gas port when a load of said boiler is low,increase a flow amount of the jet from said gas port in accordance thatthe load of said boiler becomes higher and reduce a flow amount of thejet from said gas port when said concentration of CO is equal to or lessthan a predetermined value.

[0020] The control means may be means for increasing the flow amount ofsaid jet in accordance with a lowness of a fuel ratio in a pulverizedcoal.

[0021] The supply means for supplying the gas for jetting to said gasport may be means for branching the air for combustion of said burner soas to make the air for jetting. In this case, it is preferable that aflow amount adjusting damper is provided in each of a flow passage ofthe air for combustion and a flow passage of the air for jetting.

[0022] The supply means for supplying the gas for jetting to said gasport may be means for branching the air for transferring said pulverizedcoal so as to make the air for jetting.

[0023] In the case that an after air port for a two stage combustion isplaced downstream said burner stage, the supply means for supplying thegas for jetting to said gas port can be means for branching the afterair so as to make the air for jetting.

[0024] In accordance with the present invention, since in a boilercomprising a combustion chamber formed by front and rear walls and aside wall crossing to said front and rear walls and a plural stages ofburners placed on at least one of said front and rear walls, in order tomake a pressure of a gas within said combustion chamber higher in aportion near the side wall than at a center portion of said combustionchamber, a gas port is provided between an outermost row burner and saidside wall within a range of a height of said burner stages, it ispossible to increase a pressure of the gas near the side wall so as toprevent the combustion gas from coming close to the side wall, therebyreducing an attachment of the ash due to a collision of the combustiongas, a concentration of CO at an outlet of the combustion chamber and anunburned matter.

[0025] In this case, in the embodiments which will be mentioned below, aboiler corresponds to a boiler in which a combustion gas generated by acombustion of a fuel flows from an inlet port of a fuel toward an outletport of a furnace in one direction.

BRIEF DESCRIPTION OF DRAWINGS

[0026]FIG. 1 is a perspective view which shows a summarized structure ofa furnace in an embodiment 1 of a once-through boiler in accordance withthe present invention;

[0027]FIG. 2 is a cross sectional view which shows an embodiment of astructure of a gas port in the embodiment 1;

[0028]FIG. 3 is a front elevational view which shows an embodiment of astructure of the gas port in FIG. 2;

[0029]FIG. 4 is a view which shows a summary of a stream of a combustiongas and a gas jet within the furnace in the embodiment 1 in which thegas port is placed on a front wall and a rear wall;

[0030]FIG. 5 is a front elevational view which shows a summary of astream of the combustion gas in the conventional furnace in which thegas port is not placed;

[0031]FIG. 6 is a view which shows a summary of a stream of a combustiongas and a gas jet within a furnace in accordance with an embodiment 2 inwhich a gas port is placed on a left side wall and a right side wall;

[0032]FIG. 7 is a perspective view which shows a summarized structure ofa furnace in an embodiment 3 of a once-through boiler in accordance withthe present invention;

[0033]FIG. 8 is a front elevational view which shows a stream linetoward a direction of the left side wall in the embodiment 3;

[0034]FIG. 9 is a view which shows a result of calculating aconcentration of CO (%) at a position 10 cm apart from the left sidewall of the embodiment 3;

[0035]FIG. 10 is a front elevational view which shows a stream linetoward a direction of the left side wall in the conventionalonce-through boiler;

[0036]FIG. 11 is a view which shows a result of calculating aconcentration of CO (%) at a position 10 cm apart from the left sidewall shown in FIG. 10;

[0037]FIG. 12 is a front elevational view which shows a stream linetoward a direction of the left side wall in the conventionalonce-through boiler in which an apparatus of an air flowed near boundarylayer of wall for forming a stream of an air along the wall is providedat a lower portion of the furnace;

[0038]FIG. 13 is a view which shows a result of calculating aconcentration of CO (%) at a position 10 cm apart from the left sidewall shown in FIG. 12;

[0039]FIG. 14 is a view which shows a comparison of characteristicbetween a burner A in which a stoichiometric ratio of burner is near 0.8and a value of Nitrogen Oxide at the outlet of the furnace becomes aminimum value and a burner B in which a stoichiometric ratio of burneris near 0.7 and a value of Nitrogen Oxide at the outlet of the furnacebecomes a minimum value;

[0040]FIG. 15 is a systematic view which shows a structure of anembodiment 4 of a once-through boiler in accordance with the presentinvention;

[0041]FIG. 16 is a characteristic view which shows an embodiment of arelation between a load and a flow amount of a jet from the gas port;

[0042]FIG. 17 is a characteristic view which shows an embodiment of arelation between a fuel ratio and a flow amount of a jet from the gasport;

[0043]FIG. 18 is a characteristic view which shows an embodiment of arelation between a concentration of CO and a flow amount of a jet fromthe gas port;

[0044]FIG. 19 is a side elevational view of a furnace which shows supplymeans for supplying an air for jet by branching an air for combustion ina burner;

[0045]FIG. 20 is a side elevational view of a furnace which shows supplymeans for supplying an air for jet by branching from an upstream of adamper for adjusting an air flow amount in the burner; and

[0046]FIG. 21 is a side elevational view of a furnace which shows anembodiment in which in the case that the gas port is close to the afterair port, the air for jet is branched from the after air and the airpipe is made shorter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] Next, embodiments of a once-through boiler in accordance with thepresent invention will be described below with reference to FIGS. 1 to21.

[0048] Embodiment 1

[0049]FIG. 1 is a perspective view which shows a summarized structure ofa furnace in an embodiment 1 of a once-through boiler in accordance withthe present invention. The furnace has a front wall 14 a and a rear wall14 b, and a left side wall 1 a and a right side wall 1 b crossing to thewalls 14 a and 14 b. A plurality of burners are mounted to at least oneof the opposing front wall 14 a and rear wall 14 b in a plural stagesand a plural rows. In the case of the embodiment 1, a lower stage burner2, a middle stage burner 3 and an upper stage burner 4 are respectivelyconstituted by four rows of burners. Each of the burners supplies a fueland an air for combustion to a combustion chamber 13.

[0050] A gas port arranged by the present invention is positionedbetween the lower stage burner 2 and the upper stage burner 4 in aheight direction and between a side wall 1 and an outermost row burnerin a lateral direction. The gas port 6 in the embodiment 1 is formed ata portion having the same height as that of the middle stage burner 3.The gas port 6 of the front wall 14 a and the gas port 6 of the rearwall 14 b are formed at a position at which a jet of the gas port 6collides.

[0051] In the embodiment 1, a gas not containing a fuel is supplied fromthe gas port 6. A component of the gas not containing the fuel includesan air, an oxygen, a burned exhaust gas and the like. It is notnecessary that flow speeds of the opposing jets are equal to each other,and it is possible to adjust a position at which the jets are collidedwith each other and a pressure in the colliding position when changing aflow speed and a flow amount of the jets.

[0052]FIG. 2 is a cross sectional view which shows an embodiment of astructure of the gas port 6 in accordance with the embodiment 1. FIG. 3is a front elevational view which shows the embodiment of the structureof the gas port 6 shown in FIG. 2. A shape of the gas port 6 is definedby a water tube 17 constituting a boiler. The water tube 17 is arrangedaround the gas port 6 in a direction parallel to a center axis of thegas port 6. When arranging the water tube 17 in this manner, a dampingof a jet 18 in the gas port 6 is reduced so as to increase a pressure ata time when the jet 18 is collided. An optimum shape of the gas port 6is a cylindrical shape in which a cross section is a circular shape.When the cross section of the gas port 6 is a circular shape, it is easyto bend the water tube 17 so as to form the gas port 6.

[0053]FIG. 4 is a view which shows a summary of a stream of thecombustion gas 16 and the jet 18 within the furnace of the embodiment 1in which the gas port 6 is placed in the front wall 14 a and the rearwall 14 b. When placing the gas port 6, the combustion gas 16 can notcome close to the side walls 1 a and 1 b due to the jet 18 of the gasinjected from the gas port 6. Because the pressure near the side walls 1a and 1 b becomes increased due to the jet 18 of the gas injected fromthe gas port 6.

[0054]FIG. 5 is a front elevational view which shows a summary of astream of the combustion gas in the conventional furnace in which thegas port 6 is not placed. In the case that the gas port 6 is not placed,the combustion gas 16 formed by the burner stages 2, 3 and 4 flows inthe direction of the side walls 1 a and 1 b. Since the combustion gas 16from the lower stage burner 2 is prevented by the combustion gas 16 inthe middle burner 3 and the upper burner 4 and can not ascend in animmediately upper direction, the gas 16 flows in a direction of the sidewalls 1 a and 1 b in which a pressure is low.

[0055] A certain effect can be obtained even when the gas port 6 isformed between a bottom of the furnace to a top thereof not immediatelybeside the burner stages 2, 3 and 4. However, the effect becomes smallwhen it is placed at a portion apart from the burner stages 2, 3 and 4.

[0056] As shown in the prior art, when forming the gas port 6 in a lowerside of the burner stages 2, 3 and 4, the pressure of the portion nearthe side wall 1 becomes high at the formed height, however, the pressurebecomes low at the height of the burner stages 2, 3 and 4, so that thecombustion gas 16 generated by the burners 2, 3 and 4 flows in adirection of the side walls 1 a and 1 b.

[0057] When forming the gas port 6 in an upper side of the burner stages2, 3 and 4, the pressure of the portion near the side walls 1 a and 1 bis increased in comparison with the case that the gas port 6 is notformed. However, in comparison with the case of forming the gas port inthe area of the burner stages 2, 3 and 4, an increase of the pressure isa little and the combustion gas 16 generated by the burners 2, 3 and 4easily flows in a direction of the side walls 1 a and 1 b.

[0058] The jet 18 from the gas port 6 can achieve the object of thepresent invention well when reaching the center portion of each of theside walls 1 a and 1 b. In the case that the jet 18 can not reach thecenter portion of each of the side walls 1 a and 1 b, the combustion gas16 easily flows in a direction of the side walls la and lb. Accordingly,it is necessary to collide the jet 18 in the center portion of each ofthe side walls 1 a and 1 b. A desirable flow speed of the jet 18 iswithin a range between 30 m/s and 90 m/s. Further, in the case that thegas port 6 is of the type of supplying a direct flow gas, since it ispossible to make the damping of momentum of the gas smaller than thetype of supplying a swirling flow gas, it is possible to supply the gasto the center portion of the side walls 1 a and 1 b at a higherpressure.

[0059] The jet 18 at the gas port 6 maybe not only supplied inperpendicular to the burner wall 14 but also supplied at an optionalangle. When supplying the jet 18 at the gas port 6 in such a manner asto direct to an inner portion of the combustion chamber 13, it is hardthat the combustion gas 16 flows in a direction of the side wall 1. Wheninjecting the jet 18 toward the side wall 1, the gas in the jet 18 canbe supplied along the side wall 18. When the combustion gas 16 comesclose to the side wall 1, a heat absorption of the side wall 1 isincreased, so that a temperature of a water wall constituting the sidewall 1 is increased. The jet 18 at the gas port 6 also serves to coolthe side wall 1.

[0060] Embodiment 2

[0061]FIG. 6 is a view which shows a summary of a stream of thecombustion gas 16 and the jet 18 within the furnace in accordance withan embodiment 2 in which a gas port 8 is placed on the left side wall 1a and the right side wall 1 b. The structures of the burner stages 2, 3and 4, the front wall 14 a and the rear wall 14 b are the same as thoseof the embodiment 1. It is not necessary that the gas port 8 is placedonly on the front wall 14 a and the rear wall 14 b on which the burnerstages 2, 3 and 4 are arranged. When the gas port 8 is placed on theside walls 1 a and 1 b, the same effect as that of the embodiment 1 canbe obtained. In this case, as well as the embodiment 1, it is necessaryto increase the pressure near the side walls 1 a and 1 b. It is properto set a flow speed of the jet 18 to a range between 30 m/s and 90 m/s.Further, the jet 18 at the gas port 8 may be not only supplied inperpendicular to the rear wall 14 b but also supplied at an optionalangle. FIG. 6 shows an embodiment in which the jet 18 is supplieddownward. When directing the jet 18 downward, the jet 18 and thecombustion gas 16 are collided with each other, so that the pressure isincreased. As a result, the combustion gas 16 can not come close to adirection of the side walls 1 a and 1 b.

[0062] Embodiment 3

[0063]FIG. 7 is a perspective view which shows a summarized structure ofa furnace in an embodiment 3 of a once-through boiler in accordance withthe present invention. The structures of the burner stages 2, 3 and 4,the front wall 14 a and the rear wall 14 b are the same as those of theembodiment 1. An after air port 9 for a two stage combustion is mountedto an upper portion of the burner stages 2, 3 and 4. At least one stageof gas port 6 is placed between the lower stage burner 2 and the upperstage burner 4, and a plural stages of gas ports 6 are placed betweenthe lower stage burner 2 and the after air port 9. In the embodiment 3,they are mounted at a portion having the same height as that of themiddle burner 3, between the upper burner 4 and the after air port 9 anda portion having the same height as that of the after air port 9,totally at three portion.

[0064] The jet 18 from the gas port 6 increases the pressure at thecenter portion of the side wall 1 and prevents the combustion gas 16from coming close to the side wall 1, as in the same manner as that ofthe embodiment 1. When placing the gas port 6 in the burner stages 2, 3and 4, it is hard that the combustion gas 16 comes close to the sidewalls 1 a and 1 b and at the same time the deoxidization gas generatedin accordance with a two stage combustion method is oxidized, so that itis possible to reduce the concentration of CO and the unburned matternear the side wall 1 a and 1 b. Further, the pressure near the sidewalls 1 a and 1 b is increased by placing a plural stages of gas ports 6as shown in FIG. 7, so that it is hard that the combustion gas 16containing the deoxidization gas comes close to the side wall 1.

[0065]FIG. 8 is a front elevational view which shows a stream line 41toward a direction of the left side wall 1 a in the embodiment 3. FIG. 9is a view which shows a result of calculating a concentration of CO (%)at a position of 10 cm apart from the left side wall 1 a in theembodiment 1. The numerically analyzed boiler is a boiler having amaximum output power of a pulverized coal flame of 500 MW and under astate of 100% load. 4% air for combustion is supplied from the gas port6. An injection speed is 40 m/s.

[0066]FIG. 10 is a front elevational view which shows the stream line 41toward a direction of the left side wall lain the conventionalonce-through boiler. FIG. 11 is a view which shows a result ofcalculating a concentration of CO (%) at a position of 10 cm apart fromthe left side wall 1 a shown in FIG. 10. The numerically analyzed boileris a boiler having a maximum output power of a pulverized coal flame of500 MW and under a state of 100% load.

[0067]FIG. 12 is a front elevational view which shows a stream line 41toward a direction of the left side wall 1 a in the conventionalonce-through boiler in which an apparatus of an air flowed near boundarylayer of wall 42 for forming an air flow along the wall is provided inthe lower portion of the furnace. FIG. 13 is a view which shows a resultof calculating a concentration of CO (%) at a position of 10 cm apartfrom the left side wall 1 a shown in FIG. 12. The numerically analyzedboiler is a boiler having a maximum output power of a pulverized coalflame of 500 MW and under a state of 100% load. 8% air for combustion issupplied from the apparatus of an air flowed near boundary layer of wall42 shown in FIG. 12 as an air flowed near boundary layer of wall 43.

[0068] In comparison among FIGS. 8, 10 and 12, in the case that the gasport 6 is provided in such a manner as shown in FIG. 8 on the basis ofthe embodiment 1 of the present invention, the flow of the combustiongas 16 toward a direction of the side walls 1 a and 1 b is less than theconventional embodiment shown in FIG. 12. In particular, there hardlyexists the stream toward a direction of the side walls 1 a and 1 b fromthe middle stage burner 3 and the upper stage burner 4. The jet 18 fromthe gas port 6 prevents the combustion gas 16 from colliding with theside wall. In the case that the apparatus of an air flowed near boundarylayer of wall 42 shown in FIG. 12 is provided, it is hardly possible toprevent the combustion gas 16 from colliding with the side walls 1 a and1 b.

[0069] A concentration of CO near the side wall 1 in the embodiment 1 inaccordance with the present invention shown in FIG. 9 becomes equal toor less than 1% at a portion downstream the burner stage.

[0070] A concentration of CO near the side wall 1 in the conventionaltype boiler shown in FIG. 11 attains 10% at the maximum between theupper stage burner 4 and the after air port 9. Carbon monoxide near theside wall 1 is hard to be oxidized and flows to the outlet port 5 of thefurnace.

[0071] A concentration of CO near the side wall 1 in the conventionaltype boiler in which the air flowed near boundary layer of wall 42 shownin FIG. 13 is placed is 8% at the maximum, and is hardly different fromthat of the conventional type boiler. The distribution of theconcentration of CO mentioned above is established because thecombustion gas 16 flowing from the burners 2, 3 and 4 flows in adirection of the side wall 1 having a low pressure even after flowingthe air flowed near boundary layer of wall 42 along the side wall 1,thereby colliding with the side wall 1.

[0072]FIG. 14 is a view which shows a comparison of characteristicbetween a burner A in which a stoichiometric ratio of burner is near 0.8and a value of Nitrogen Oxide at the outlet 5 of the furnace becomes aminimum value and a burner B in which a stoichiometric ratio of burneris near 0.7 and a value of Nitrogen Oxide at the outlet 5 of the furnacebecomes a minimum value. It is desirable that the burner used in theembodiment 3 has a characteristic that a value of Nitrogen Oxide at theoutlet 5 of the furnace becomes a minimum value under an operationcondition such that a stoichiometric ratio of burner is lower than 0.8.When using the burner B, in order to reduce Nitrogen Oxide at the outlet5 of the furnace, it is effective to reduce the stoichiometric ratio ofburner to 0.7 rather than 0.8. However, when lowering the stoichiometricratio of burner, the deoxidization gas generated in the burner stages 2,3 and 4 flows near the side wall 1, thereby increasing the concentrationof CO and the unburned matter.

[0073] Accordingly, the conventional type boiler has been operated undera condition of the stoichiometric ratio of burner of about 0.8, andNitrogen Oxide at the outlet 5 of the furnace has been substantially thesame between the burner A and the burner B.

[0074] On the contrary, in accordance with the present invention, forexample, since it is possible to reduce the concentration of CO and theunburned matter near the side wall 1 when placing the gas port 6 asshown in the embodiment 3, it is possible to use the burner B in whichthe stoichiometric ratio of burner is near 0.7 and a value of NitrogenOxide at the outlet 5 of the furnace becomes a minimum value, so that incomparison with the case of using the burner A, it is possible to reduceNitrogen Oxide at the outlet 5 of the furnace.

[0075] Embodiment 4

[0076]FIG. 15 is a systematic view which shows a structure of anembodiment 4 of a once-through boiler in accordance with the presentinvention. A used fuel is a coal 23 and is stored in a coal bunker 37. Acoal stored in the coal bunker 37 is pulverized by a coal pulverizer 38.A coal feeding air 33 and the coal are supplied to a burner 39. An airsupplied from a blower 31 is heated by a burned exhaust gas 32 and anair heater 30. The heated air is separated into a coal feeding air 34,an air for combustion 35 and an air for jet 36 at the gas port 6. Adamper 27 and a flow amount meter 26 are placed in pipes for the coalfeeding air 34, the air for combustion 35 and the air for jet 36. Acontrol apparatus 20 inputs a load demand 21, a coal type information22, a coal type measuring result 24 and a flow amount signal 25 of theair for jet 36 so as to control a damper 27 of the air for jet 36. It issufficient that the gas port 6 is placed in such a manner as shown inthe embodiment 1 or the embodiment 2.

[0077] The control apparatus 20 estimates a characteristic of the coalon the basis of the coal type information 22 or the coal type measuringresult 24, controls an opening degree of the damper 27 in response tothe estimated coal characteristic, the load demand and the flow amount25 of the air for jet 36, and adjusts a jet 18 from the gas port 6.

[0078]FIG. 16 is a characteristic view which shows an example of arelation between the load and the flow amount of the jet 18 from the gasport 6. Since the pressure of the combustion area within the furnace isnot high when the load is low, a flow amount of the combustion gas 16flowing in a direction of the side wall 1 is a little. Accordingly, theflow amount of the jet 18 from the gas port 6 is set to be a little. Asthe load becomes higher, the flow mount of the jet 18 from the gas port6 is set to be increased.

[0079]FIG. 17 is a characteristic view which shows an example of arelation between a fuel ratio and the flow amount of the jet 18 from thegas port 6. In the case of a coal having a low fuel ratio, since anamount of a deoxidization gas in the combustion gas 16 flowing in adirection of the side wall 1 is increased, the flow amount of the jet 18from the gas port 6 is set to be increased. On the contrary, in the caseof a coal having a high fuel ratio, since a combustion is not promotedand the amount of the deoxidization gas is reduced in comparison withthe coal having a low fuel ratio, the flow amount of the jet 18 from thegas port 6 is set to be reduced.

[0080] When setting the flow amount of the jet 18 at the gas port 6 tobe minimum without breaking the deoxidization area formed within thefurnace in accordance with the control method shown in FIGS. 16 or 17,it is possible to maintain a concentration of Nitrogen Oxide at theoutlet 5 of the furnace to be always minimum.

[0081]FIG. 18 is a characteristic view which shows an example of arelation between the concentration of CO and the flow amount of the jet18 from the gas port 6. Without the coal information 22 or the coal typemeasurement result 24, it is possible to mount a CO concentrationmeasuring apparatus 28 to, for example, the side wall 1 so as to takeinto a CO concentration signal 29 and control the flow amount of the jet18 from the gas port 6 in accordance with the concentration of CO. Inthis case, when the CO concentration signal 29 is equal to or more thanabout 4% as shown in FIG. 18, the damper 27 is opened so as to increasethe flow amount of the jet 18 at the gas port 6. In the case that theconcentration of CO 29 is equal to or less than 4%, the damper 27 isclosed so as to reduce the flow amount of the jet 18 at the gas port 6.As is apparent from the distribution of the concentration of CO shown inFIG. 9 mentioned above, it is not necessary to limit the concentrationof CO for starting the control to 4%. That is, when the concentration ofCO is equal to or less than 4% near the burners 2, 3 and 4, it isconsidered that a flame does not collide with the side wall 1, so thatit is possible to select an optional concentration of CO between 0 and4%.

[0082] Embodiment 5

[0083]FIGS. 19, 20 and 21 are side elevational views which showvariations of supply means for supplying the air for jet 36 to a furnace15.

[0084] The air for jet 36 shown in FIG. 19 is supplied by branching theair for combustion 35 of the burner 39. Since the pressure of the airfor combustion 35 of the burner is high, it is possible to inject thejet 18 at a high speed, so that it is preferable for increasing thepressure near the side wall 1.

[0085] The air for jet 36 shown in FIG. 20 is branched from an upstreamof the damper 27 for adjusting the air flow amount of the burner 39.When branching the air for jet 36 in a manner mentioned above, thepressure of the air for jet 36 is a little changed even by changing theflow amount of the air for combustion to the burner 39, so that it ispossible to inject the air for jet 36 at a further high speed. Further,it is possible to independently control the air for jet 36 and the airfor combustion in the burner 39.

[0086]FIG. 21 shows an embodiment in which in the case that the gas port6 is close to the after air port 9, the air for jet 36 is branched fromthe after air 45 and the air pipe is made shorter.

[0087] In the conventional embodiment disclosed in Japanese PatentUnexamined Publication No. 7-98103 mentioned above, the pipe forsupplying the gas for combustion having an oxygen partial pressure of10% or less to the auxiliary combustion port was necessary. Accordingly,it is necessary to arrange the pipe for supplying the gas for combustionhaving a length of some tens meters, so that a large cost increase wasunavoidable.

[0088] On the contrary, in the supply means for supplying the air forjet 36 to the furnace 15 in accordance with the present invention asshown in FIGS. 19, 20 and 21, it is sufficient to only branch the airfor combustion 35 or the after air 45 piped to a very near position soas to supply the air for jet 36. In particular, in the case that the gasport 6 is provided at the same height as that of the burner stages 2, 3and 4, since it is possible to form the gas port 6 at both right andleft ends of a window box 40 in the burner 39, it is sufficient to addonly a minimum number of equipment for the present invention. In thecase that the gas port 6 is provided at the same height as that of theafter air port 9, the same matter can be also applied.

[0089] In accordance with the present invention, since in a once-throughboiler comprising a combustion chamber formed by front and rear wallsand a side wall crossing to said front and rear walls and a pluralstages of burners placed on at least one of said front and rear walls, agas port is provided between an outermost row burner and said side wallwithin a range of a height of said burner stages so as to inject a gasinto the combustion chamber, thereby making a pressure of a gas near theside wall higher than a pressure of a gas at a center portion of thecombustion chamber, it is possible to prevent the combustion gas fromcoming close to the side wall, thereby reducing an attachment of the ashdue to a collision of the combustion gas, a concentration of CO at anoutlet of the furnace and an unburned matter.

What is claimed is:
 1. A boiler comprising a combustion chamber formedby front and rear walls and a side wall crossing to said front and rearwalls and a plural stages of burners placed on at least one of saidfront and rear walls, wherein a gas port is provided between anoutermost row burner and said side wall within a range of a height ofsaid burner stages.
 2. A boiler comprising a combustion chamber formedby front and rear walls and a side wall crossing to said front and rearwalls and a plural stages of burners placed on at least one of saidfront and rear walls, wherein a gas jet port for making a pressure of agas near said side wall within said combustion chamber higher than apressure of a gas at a center portion of said combustion chamber isprovided in said side wall within a range of a height of said burnerstages.
 3. A boiler comprising a combustion chamber formed by front andrear walls and a side wall crossing to said front and rear walls, aplural stages of burners placed on at least one of said front and rearwalls and an after air port for a two stage combustion disposeddownstream said burner stages, wherein at least one stage gas jet portfor making a pressure of a gas near said side wall within saidcombustion chamber higher than a pressure of a gas at a center portionof said combustion chamber is provided between an outermost row burnerand said side wall within a range of a height of said burner stages anda plural stages of gas jet ports are provided between said lowermoststage burner and said after air port.
 4. A boiler as claimed in claim 1, wherein said gas port is provided at portions of said opposing frontand rear walls, said portions having the same height, and wherein gassupply means for injecting said jet at a speed at which a gas jet fromsaid opposing gas port collides in the middle of said front and rearwalls is provided.
 5. A boiler as claimed in claim 1 , comprising supplymeans for supplying a pulverized coal as a fuel and an air fortransferring said pulverized coal to said plural stages of burners, andsupply means for supplying an air for combustion to said plural stagesof burners and supply means for supplying a gas for jetting to said gasport, wherein there is provided control means for controlling a flowamount of the jet from said gas port on the basis of a load demand ofsaid boiler and a coal type information so as to reduce a flow amount ofthe jet from said gas port when a load of said boiler is low andincrease a flow amount of the jet from said gas port in accordance thatthe load of said boiler becomes higher.
 6. A boiler as claimed in claim1 , comprising supply means for supplying a pulverized coal as a fueland an air for transferring said pulverized coal to said plural stagesof burners, and supply means for supplying an air for combustion to saidplural stages of burners and supply means for supplying a gas forjetting to said gas port, wherein measurement means for measuring aconcentration of a carbon monoxide (CO) in a combustion gas near saidside wall is provided, and there is provided control means forcontrolling a flow amount of the jet from said gas port on the basis ofa load demand of said boiler and a measured result of said concentrationof CO so as to reduce a flow amount of the jet from said gas port when aload of said boiler is low, increase a flow amount of the jet from saidgas port in accordance that the load of said boiler becomes higher andreduce a flow amount of the jet from said gas port when saidconcentration of CO is equal to or less than a predetermined value.
 7. Aboiler as claimed in claim 5 , wherein said control means is means forincreasing the flow amount of said jet in accordance with a lowness of afuel ratio in a pulverized coal.
 8. A boiler as claimed in claim 5 ,wherein said supply means for supplying the gas for jetting to said gasport is means for branching the air for combustion of said burner so asto make the air for jetting.
 9. A boiler as claimed in claim 8 , whereina flow amount adjusting damper is provided in each of a flow passage ofthe air for combustion and a flow passage of the air for jetting.
 10. Aboiler as claimed in claim 5 , wherein said supply means for supplyingthe gas for jetting to said gas port is means for branching the air fortransferring said pulverized coal so as to make the air for jetting. 11.A boiler as claimed in claim 5 , comprising an after air port for a twostage combustion placed downstream said burner stage, wherein the supplymeans for supplying the gas for jetting to said gas port is means forbranching the after air so as to make the air for jetting.